In general, an optical film, especially a retardation film is used in displays such as liquid crystal displays and has such functions as color correction, view angle expansion and antireflection.
A λ/4 plate and a λ/2 plate are known as the retardation film, and thermoplastic polymers such as polycarbonate obtained by polycondensing bisphenol A, polyether sulfone and polysulfone are used as the materials of these plates. The λ/4 plate and the λ/2 plate obtained by stretching films of these materials are characterized in that their retardation becomes larger as the wavelength becomes shorter. Therefore, the wavelength at which the λ/4 plate and the λ/2 plate can function is limited to a specific wavelength.
As means of controlling the wavelength at a broad band, there is known a method in which two or more birefringence films having different wavelength dependences of retardation are laminated together at a specific angle (refer to Patent Document 1). In this case, as a plurality of retardation films are used, the step of laminating together these retardation films and the step of adjusting the lamination angle are required, thereby causing a problem with productivity. Further, as the thickness of the whole retardation film becomes large, its light transmittance lowers, whereby when it is set in an apparatus, the apparatus becomes thick and dark.
There has recently been proposed a method for expanding the bandwidth with a single film without lamination (refer to Patent Document 2). This film is obtained by stretching a polymer film comprising a unit having positive refractive index anisotropy and a unit having negative refractive index anisotropy. However, as the film is a polymer film comprising a unit having positive refractive index anisotropy and a unit having negative refractive index anisotropy, it is difficult to make it thin due to its low birefringence. It also has a problem that its contrast is not good due to its low Nz coefficient. Further, as an aromatic copolycarbonate having a fluorene-based bisphenol skeleton is used, it has a high melting temperature, thereby readily producing a gelled product through its decomposition when it is melt processed. Further, as it has a high Tg (glass transition temperature), a high temperature is required for stretching the film, and special processing equipment different from that of the prior art is needed. Therefore, it cannot be said that its processability is satisfactory. Further, as the film has a high photoelastic constant, its stress birefringence becomes large and light slipping occurs when the film is used as a retardation film. It is known that this phenomenon is proportionate to the absolute value of “photoelastic constant× thickness”, and it is therefore desired that the photoelastic constant should be low and the thickness should be small.
To produce a retardation film having a reverse wavelength dispersion to reduce its photoelastic constant, there is reported a method in which a polyvinyl acetal resin containing a specific structure is melt extruded into a sheet form and then stretched (refer to Patent Document 3). However, this method has problems that it is difficult to make the film thin due to its low birefringence and its Nz coefficient is low.
(Patent Document 1) JP-A 02-120804
(Patent Document 2) Japanese Patent No. 3325560
(Patent Document 3) JP-A 2006-234878